The invention described herein arose in the course of, or under, Contract No. W-7405-ENG-48 between the United States Department of Energy and the University of California.
In the operation of an interferometer such as a Fabry Perot interferometer, it is known that the light output can be increased by as much as 50 times by removing a portion of reflective material from the input mirror surface in the form of a narrow stripe of, for example, .about.700 micrometers width.
However, it has been found to be very difficult to accurately remove such a narrow width of reflective material in a manner which will not damage the remainder of the mirror, yet provide a stripe of uniform width with little or no residues remaining, in the stripe, from the removal of the reflective material.
For example, masking and etching techniques in common usage in the integrated circuit chip industry can form very accurate and fine lines or openings on mirrors with hard coatings of dielectric reflective materials. However the process of applying such hard coatings to mirrors induces stress and distortion into the underlying glass, resulting in degradation of the optical properties of the mirror.
Ion etching of either hard or soft dielectric reflective coatings is difficult to control, both from the standpoint of the desired anisotropy of the etch, as well as control of the depth of the etch, i.e., to remove only the reflective dielectric coating material without also etching the underlying glass.
We have, therefore, found that mechanical scoring or cutting of soft dielectric reflective material, e.g., reflective material such as a zinc sulfide/magnesium fluoride layered dielectric reflective material sensitive to light of visible wavelengths (450-650 nm) , available from the Burleigh Instruments Company, is the only feasible means for forming the stripe without damaging the mirror. However, such soft dielectric material may be very easily destroyed, i.e., by merely touching the surface of the coating of soft reflective material on the glass surface.
Therefore, any mechanical removal means must be capable of performing a sharp and accurate straight edge cut without touching other parts of the coating so that the remainder of the reflective surface will not be damaged during the formation of an accurate stripe of uniform width, so that the amount of light transmitted through the clear stripe (from the backside of the mirror) will be consistent along the entire length of the stripe.